I just got done replacing a stripped worm-follower gear on my KA 600 for the second time in 6 months. After the first one stripped, I made sure to stay at speed 2 or below with dough, and still it failed. The mixer is only 2 years old and I just really got into bread and pizza about a year ago.
I just got a new bosch a couple weeks ago and it is immensely better than the KA. Although, I haven't used it for cookies/cakes or anything like that.
My dilemma with the KA is that I also own the meat grinder and a very expensive pasta roller set with it. So, I repaired the KA and am thinking of keeping it just for light duty work and using the attachments. If it fails again, I will probably repair it one more time and try to dump it on ebay.
I would strongly, strongly recommend going with the Bosch. It doesn't even break a sweat working dough.
Also note with the KA, they give you a 1 year warranty and then flood you with mail notices offering to sell you an extended warranty. Ridiculous for such an expensive machine. I railed at their customer support, but they just said "too bad, so sad" ... not exactly but for all practical purposes.
You can buy the replacement gears online pretty cheap and also need to plunk down $20 for the food grade grease. All told, it will cost you around $20 to $35 to repair a gear each time it fails.